The present invention relates to digital printing and, more particularly, to a dot dispersion technique to improve tone reproduction in an ink jet printer.
Ink jet printing systems are known in which a print head defines one or more rows of orifices which receive an electrically conductive recording fluid, such as for instance a water based ink, from a pressurized fluid supply manifold and eject the fluid in rows of parallel streams. Printers using such print heads accomplish graphic reproduction by selectively charging and deflecting the drops in each of the streams and depositing at least some of the drops on a print receiving medium, while others of the drops strike a drop catcher device.
The various techniques for digital printers to provide continuous tone (monochrome or color) include ordered drop dispersion patterns, the building of dot clusters (which look similar to conventional offset halftone printing and are typically printed with very high resolution digital printers such as 1200 dpi), and stochastic (error diffusion) screening. An example of simple drop dispersion is the Bayer fill sequence, known in the art. Other examples are embedded within the Adobe program, PhotoShop. That program also includes the dot clustering technique with selectable lines/inch and optional screening angles. Error diffusion has been available for over a decade and has recently been incorporated into many of the desk top printers.
Unfortunately, many dispersion patterns produce objectionable texture or artifacts, especially in the highlights. In addition, some halftone algorithms for color printing require more accurate registration of the different color dots than is possible with web printers, resulting in unpredictable color shifts. Dot clusters (combined with screen angles) try to resolve this problem, but the graininess is very bad. Error diffusion provides the best results, but is pattern-dependent and very math-intensive; on-line rendering of images cannot keep up with the high speed printers being developed.
Fundamentally, ordered dithers (angles, Bayer, etc.) have been employed in the past to permit gray scale printing from limited bit depth imaging systems. More recently, random dithers have been developed to hide the patterning. In general, these random dithers hide repeatable artifacts as they grow in size. At a 256xc3x97256 matrix size, the patterns appear to be random to the eye. However, the quality is still not as good as error diffusion, causing images to appear fuzzy, snowy or grainy. The reason for this effect is tied to the size of the matrix. Representing a midtone in a matrix that large causes holes and dense spots inside of the random matrix. This randomization is reflected in large areas of homogenous tone and in edge definition. Error diffusion is better because it optimizes each pixel on an individual basis, so the images remain sharp and smooth. Even so, images created with error diffusion have visible image artifacts in the midtone range that range from annoying to severe.
It would be desirable then to be able to provide continuous tone reproduction, or images, using improved ink jet droplet dispersion techniques that eliminate visible image artifacts.
The present invention proposes the use of custom tuned random dithers in the highlights that transition into structured dithers in the midtone and shadow areas. This takes advantage of the benefits of random dithers while eliminating their negative effects. To accomplish this, the present invention proposes using two distinct dot placement schemes for dot dispersion. In the light areas, having little ink coverage, ink dots are dispersed pseudo-randomly in the halftone cell. In the medium to high density tones, the pseudo-random patterns can give visibly annoying artifacts, so these coverage areas apply an ordered pattern.
In accordance with one aspect of the present invention, continuous tone reproduction is provided through improved ink jet droplet dispersion techniques. In the highlights, such as the range from no dots up through a transition zone, for example, approximately 25% coverage in an ink jet system, (25% of the pixels have a dot or drop of ink), the dot placement scheme disperses ink dots pseudo-randomly in the halftone cell. Of course, in other systems, the transition zone may be at a higher or lower coverage level. Beyond the 25% range, the dot placement scheme disperses ink dots in an ordered pattern. Of course, the 25% point of cross-over is arbitrary and could be any coverage value, such as 15% or 50%, and will vary depending on the printing system being used. The purpose of the transition zone is to minimize visual artifacts between dither patterning techniques of the present invention, and the transition zone is selected to achieve this purpose. For example, within the highlight range, dots are randomly selected from only those dots used at the cross-over intensity, to assure a smooth transition from the random phase to the ordered. Beyond the highlight range, dot selection is ordered.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.